Air duct



FIG. 1 is a bottom, front, and right side perspective view of an air duct showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a left side elevational view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom view thereof. 

The ornamental design for an air duct, as shown. 